St. Francis of Assisi, by Jusepe de Ribera (1642)
[now available only in my book, Biblical Catholic Salvation: "Faith Working Through Love"]
1 Peter 3:20-21 who formerly did not obey, when God's patience waited in the days of Noah, during the building of the ark, in which a few, that is, eight persons, were saved through water.  Baptism, which corresponds to this, now saves you, not as a removal of dirt from the body but as an appeal to God for a clear conscience, through the resurrection of Jesus Christ,
Mark 16:16 He who believes and is baptized will be saved . . . [disputed biblical manuscript, but still indicative of apostolic belief]
John 6:50-51 This is the bread which comes down from heaven, that a man may eat of it and not die.  I am the living bread which came down from heaven; if any one eats of this bread, he will live for ever; and the bread which I shall give for the life of the world is my flesh."
John 6:53-58 So Jesus said to them, "Truly, truly, I say to you, unless you eat the flesh of the Son of man and drink his blood, you have no life in you;  he who eats my flesh and drinks my blood has eternal life, and I will raise him up at the last day.  For my flesh is food indeed, and my blood is drink indeed.  He who eats my flesh and drinks my blood abides in me, and I in him.  As the living Father sent me, and I live because of the Father, so he who eats me will live because of me.  This is the bread which came down from heaven, not such as the fathers ate and died; he who eats this bread will live for ever."
Acts 2:38, 40 And Peter said to them, "Repent, and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins; and you shall receive the gift of the Holy Spirit. . . .  And he testified with many other words and exhorted them, saying, "Save yourselves from this crooked generation."
Acts 22:16 And now why do you wait? Rise and be baptized, and wash away your sins, calling on his name.'
Romans 6:4 We were buried therefore with him by baptism into death, so that as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life.
Galatians 3:27 For as many of you as were baptized into Christ have put on Christ.
Titus 3:5 he saved us, not because of deeds done by us in righteousness, but in virtue of his own mercy, by the washing of regeneration and renewal in the Holy Spirit,
1 Peter 3:21 Baptism, which corresponds to this, now saves you, not as a removal of dirt from the body but as an appeal to God for a clear conscience, through the resurrection of Jesus Christ,
Mark 16:16 (RCV) He who believes and is baptized shows that he is already saved . . . [disputed biblical manuscript, but still indicative of apostolic belief]
John 6:50-51 (RCV) This is the bread which comes down from heaven, that a man may eat of it and signify that he is already in a state in which he would not die.  I am the living bread which came down from heaven; if any one eats of this bread, he proves that that he is already in a state in which he will live for ever; and the bread which I shall give for the life of the world is my flesh."
John 6:53-58 (RCV) So Jesus said to them, "Truly, truly, I say to you, unless you eat the flesh of the Son of man and drink his blood, you can't give testimony that you already have life in you;  he who eats my flesh and drinks my blood shows that he already had eternal life, and that I was already going to raise him up at the last day.  For my flesh is food indeed, and my blood is drink indeed.  He who eats my flesh and drinks my blood signifies that he already had been abiding in me, and I in him.  As the living Father sent me, and I live because of the Father, so he who eats me was already living because of me.  This is the bread which came down from heaven, not such as the fathers ate and died; he who eats this bread gives testimony that he already was living for ever."
Acts 2:38, 40 (RCV) And Peter said to them, "Repent, and be baptized every one of you in the name of Jesus Christ, in order to show forth a seal of the already existing forgiveness of your sins; and your prior gift of the Holy Spirit. . . .  And he testified with many other words and exhorted them, saying, "Give sign and testimony by baptism that you have already saved yourselves from this crooked generation."
Acts 22:16 (RCV) And now why do you wait? Rise and be baptized, and show that you have already washed away your sins, calling on his name.'
Romans 6:4 (RCV) We were buried therefore with him by baptism into death, so that as Christ was raised from the dead by the glory of the Father, we too signify that we already walk in newness of life, which is why we are being baptized.
Galatians 3:27 (RCV) For as many of you as were baptized into Christ as a seal to prove that you put on Christ before you were baptized.
Titus 3:5 (RCV) he saved us, not because of deeds done by us in righteousness, but in virtue of his own mercy, and not by the washing of regeneration, but by the renewal in the Holy Spirit,
1 Peter 3:21 (RCV) Baptism, which corresponds to this, now proves that you are already saved, not as a removal of dirt from the body but as a seal and an appeal to God for a clear conscience, through the resurrection of Jesus Christ,
The Apostle says (Colossians 2:11-12): "You are circumcised with circumcision, not made by hand in despoiling the body of the flesh, but in the circumcision of Christ, buried with Him in Baptism."
. . . Baptism is called the Sacrament of Faith; in so far, to wit, as in Baptism man makes a profession of faith, and by Baptism is aggregated to the congregation of the faithful. Now our faith is the same as that of the Fathers of old, according to the Apostle (2 Corinthians 4:13): "Having the same spirit of faith . . . we . . . believe." But circumcision was a protestation of faith; wherefore by circumcision also men of old were aggregated to the body of the faithful. Consequently, it is manifest that circumcision was a preparation for Baptism and a figure thereof, forasmuch as "all things happened" to the Fathers of old "in figure" (1 Corinthians 10:11); just as their faith regarded things to come.
. . . Circumcision was like Baptism as to the spiritual effect of the latter. For just as circumcision removed a carnal pellicule, so Baptism despoils man of carnal behavior.
. . . The protecting pillar of cloud and the crossing of the Red Sea were indeed figures of our Baptism, whereby we are born again of water, signified by the Red Sea; and of the Holy Ghost, signified by the pillar of cloud: yet man did not make, by means of these, a profession of faith, as by circumcision; so that these two things were figures but not sacraments. But circumcision was a sacrament, and a preparation for Baptism; although less clearly figurative of Baptism, as to externals, than the aforesaid. And for this reason the Apostle mentions them rather than circumcision.
(Summa Theologica, Third Part, Q 70: Circumcision; Article 1. Whether circumcision was a preparation for, and a figure of Baptism?)
Paul wanted Timothy to accompany him; and he took him and circumcised him because of the Jews that were in those places, for they all knew that his father was a Greek.Some have argued that this was hypocritical on Paul's part in much the same way as Peter was acting hypocritically in the famous incident where Paul rebuked him (Gal 2:11). How great an irony, if in fact that is true!
1127 Celebrated worthily in faith, the sacraments confer the grace that they signify. They are efficacious because in them Christ himself is at work: it is he who baptizes, he who acts in his sacraments in order to communicate the grace that each sacrament signifies. The Father always hears the prayer of his Son's Church which, in the epiclesis of each sacrament, expresses her faith in the power of the Spirit. As fire transforms into itself everything it touches, so the Holy Spirit transforms into the divine life whatever is subjected to his power.
1128 This is the meaning of the Church's affirmation that the sacraments act ex opere operato (literally: "by the very fact of the action's being performed"), i.e., by virtue of the saving work of Christ, accomplished once for all. It follows that "the sacrament is not wrought by the righteousness of either the celebrant or the recipient, but by the power of God." [footnote: St. Thomas Aquinas, S Th, III, 68, 8] From the moment that a sacrament is celebrated in accordance with the intention of the Church, the power of Christ and his Spirit acts in and through it, independently of the personal holiness of the minister. Nevertheless, the fruits of the sacraments also depend on the disposition of the one who receives them.
1129 The Church affirms that for believers the sacraments of the New Covenant are necessary for salvation. "Sacramental grace" is the grace of the Holy Spirit, given by Christ and proper to each sacrament. The Spirit heals and transforms those who receive him by conforming them to the Son of God. The fruit of the sacramental life is that the Spirit of adoption makes the faithful partakers in the divine nature [footnote: cf. 2 Peter 1:4] by uniting them in a living union with the only Son, the Savior.
Baptism consists not in the merits of those by whom it is administered, nor of those to whom it is administered, but in its own sanctity and truth, on account of Him who instituted it.
(Cont. Cres., IV)
Whence this great power of water, that it touches the body and cleanses the soul?
(Tractate 80 on the Gospel of John)
To my mind it is abundantly clear that in the matter of baptism we have to consider not who he is that gives it, but what it is that he gives; not who he is that receives, but what it is that he receives . . . Wherefore, any one who is on the side of the devil cannot defile the sacrament, which is of Christ . . . When baptism is administered by the words of the gospel, however great the evil of either minister or recipient may be, the sacrament itself is holy on account of the one whose sacrament it is. In the case of people who receive baptism from an evil person, if they do not receive the perverseness of the minister but the holiness of the mystery, being united to the church in good faith and hope and charity, they will receive the forgiveness of their sins.
(On Baptism; cited by Protestant historian Alister McGrath, Historical Theology: An Introduction to the History of Christian Thought, Wiley-Blackwell, 1998, pp. 77-78)
To show more fully the agreement between the doctrine of the Papists and that which Paul opposes, it must be observed, that the sacraments, when we partake of them in a sincere manner, are not the works of men, but of God. In baptism or the Lord’s supper, we do nothing but present ourselves to God, in order to receive his grace. Baptism, viewed in regard to us, is a passive work: we bring nothing to it but faith; and all that belongs to it is laid up in Christ. But what are the views of the Papists? They contrive the opus operatum, by which men merit the grace of God; and what is this, but to extinguish utterly the truth of the sacrament?
(Commentary on Galatians 5:1-6, section 3; translated by John King)
From the standpoint of medieval theology, Zwingli and Calvin placed the baptism of Jesus and John on the same level, partly by raising the baptism of John and partly by lowering the baptism of Christ. They elevated the baptism of John by insisting that John preached the gospel and offered the same baptism as the apostles. They lowered the baptism of Christ by arguing that it conferred no grace ex opere operato.
(Calvin in Context, Oxford University Press, 1995, p. 168)
Why, then, did the Reformers so unanimously reject ex opere operato? . . .
It is striking that so much agreement exists between Lutherans and Reformed precisely in the rejection of ex opere operato. . . . he [Calvin] objects to the ex opere operato not only because it is incorrect but because (as he remarks) it contradicts the very nature of the sacraments. . . .
[Dave: Calvin writes in IV, 14, 26 (cross-reference cited by Berkouwer, but in Latin footnotes):
It is here proper to remind the reader, that all the trifling talk of the sophists concerning the opus operatum, is not only false, but repugnant to the very nature of sacraments, which God appointed in order that believers, who are void and in want of all good, might bring nothing of their own, but simply beg. Hence it follows, that in receiving them they do nothing which deserves praise, and that in this action (which in respect of them is merely passive) no work can be ascribed to them.]
. . . we must now recognize that the Roman Catholic not only rejects this reproach of magic, but that he also faces a problem of subjectivity in the sacraments. This is already apparent in the pronouncement of Trent, which not only poses the ex opere operato, but also speaks of the problem of the obstacle. It is impossible, therefore, to speak simplistically of the Roman Catholic sacramental doctrine as "magical." . . . a subjective disposition is necessary for the working of the sacrament. Rome never intended to rule out this disposition in an objectivistic manner, but only to deny that this necessary disposition is either causal or meritorious. . . . In spite of all the criticism from the Reformed side, Rome wants to defend the gratuity of grace. . . .
This mode does not simply pit objectivity against subjectivism, nor sacrament-magic against human activity. It does not place the absolute gratuity of grace in opposition to the meritoriousness and the preparation of man. It rather synthesizes and connects these contradictory elements, and precisely in so doing it places itself against the Reformed doctrine of the sacraments. . . .
For the Reformation, the objectivity of the sacraments could no longer depend on the efficacy of infused supernatural grace . . . The sacraments are no longer independent new fountains of grace . . .
The sacrament no longer has the function of infusing supernatural grace, but can only be understood in connection with the word of promise. . . . There is a receiving of the sacrament which is altogether different from the receiving of supernatural grace.
(Studies in Dogmatics: The Sacraments, Grand Rapids, Michigan: Eerdmans, 1969, pp. 64-65, 67, 69, 74, 76)
Luther and his early followers rejected this conception of the sacraments. They do not cause grace, but are merely "signs and testimonies of God's good will towards us" (Augsburg Confessions); they excite faith, and faith (fiduciary) causes justification. Calvinists and Presbyterians hold substantially the same doctrine. Zwinglius lowered still further the dignity of the sacraments, making them signs not of God's fidelity but of our fidelity. By receiving the sacraments we manifest faith in Christ: they are merely the badges of our profession and the pledges of our fidelity.
Fundamentally all these errors arise from Luther's newly-invented theory of righteousness, i.e. the doctrine of justification by faith alone (see GRACE). If man is to be sanctified not by an interior renovation through grace which will blot out his sins, but by an extrinsic imputation through the merits of Christ, which will cover his soul as a cloak, there is no place for signs that cause grace, and those used can have no other purpose than to excite faith in the Saviour. . . .
Against all innovators the Council of Trent declared: "If anyone say that the sacraments of the New Law do not contain the grace which they signify, or that they do not confer grace on those who place no obstacle to the same, let him be anathema" (Sess. viii, can.vi). "If anyone say that grace is not conferred by the sacraments ex opere operato but that faith in God's promises is alone sufficient for obtaining grace, let him be anathema" (ibid., can. viii; cf. can. iv, v, vii).
The phrase "ex opere operato", for which there is no equivalent in English, probably was used for the first time by Peter of Poitiers (d. 1205), and afterwards by Innocent III (d. 1216; de myst. missae, III, v), and by St. Thomas (d. 1274; IV Sent., dist. 1, Q.i, a.5). It was happily invented to express a truth that had always been taught and had been introduced without objection. . . . "Ex opere operato", i.e. by virtue of the action, means that the efficacy of the action of the sacraments does not depend on anything human, but solely on the will of God as expressed by Christ's institution and promise.
"Ex opere operantis", i.e. by reason of the agent, would mean that the action of the sacraments depended on the worthiness either of the minister or of the recipient . . . It is well known that Catholics teach that the sacraments are only the instrumental, not the principal, causes of grace.
Neither can it be claimed that the phrase adopted by the council does away with all dispositions necessary on the part of the recipient, the sacraments acting like infallible charms causing grace in those who are ill-disposed or in grievous sin. The fathers of the council were careful to note that there must be no obstacle to grace on the part of the recipients, who must receive them rite, i.e. rightly and worthily; and they declare it a calumny to assert that they require no previous dispositions (Sess. XIV, de poenit., cap.4).
Dispositions are required to prepare the subject, but they are a condition (conditio sine qua non), not the causes, of the grace conferred. In this case the sacraments differ from the sacramentals, which may cause grace ex opere operantis, i.e. by reason of the prayers of the Church or the good, pious sentiments of those who use them.
Acts 2:40-41 And with many other words did he testify and exhort, saying, Save yourselves from this untoward generation.  Then they that gladly received his word were baptized: and the same day there were added unto them about three thousand souls.
Philippians 2:12-13 Wherefore, my beloved, as ye have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling.  For it is God which worketh in you both to will and to do of his good pleasure.
1 Timothy 4:16 Take heed unto thyself, and unto the doctrine; continue in them: for in doing this thou shalt both save thyself, and them that hear thee. (RSV: “Take heed to yourself and to your teaching: hold to that, for by so doing you will save both yourself and your hearers”)
Matthew 5:11-12: “’Blessed are you when men revile you and persecute you and utter all kinds of evil against you falsely on my account. Rejoice and be glad, for your reward is great in heaven, for so men persecuted the prophets who were before you.’” (cf. Mk. 9:41; Jas. 1:12; Rev. 2:10, 3:11-12)
Matthew 19:29: “’And every one who has left houses or brothers or sisters or father or mother or children or lands, for my name's sake, will receive a hundredfold, and inherit eternal life.’” (cf. 19:21)
Luke 6:38: “’give, and it will be given to you; good measure, pressed down, shaken together, running over, will be put into your lap. For the measure you give will be the measure you get back.’” (cf. 6:35; Col. 3:23-24)
1 Corinthians 3:6-9: “I planted, Apol'los watered, but God gave the growth. So neither he who plants nor he who waters is anything, but only God who gives the growth. He who plants and he who waters are equal, and each shall receive his wages according to his labor. For we are God's fellow workers; you are God's field, God's building.” (cf. 3:14; 2 Cor. 9:6; 2 Tim. 4:8)
2 Corinthians 6:1: “Working together with him, then, we entreat you not to accept the grace of God in vain.”
Ephesians 6:8: “knowing that whatever good any one does, he will receive the same again from the Lord, whether he is a slave or free.” (cf. Matt. 16:27)
Hebrews 10:35: “Therefore do not throw away your confidence, which has a great reward.” (cf. 6:10; Matt. 20:4; 2 John 8)
But in another passage, when he would remind them what the power of the word is in itself, when preached by man, he compares ministers to husbandmen, who, after they have expended labour and industry in cultivating the ground, have nothing more that they can do. For what would ploughing, and sowing, and watering avail, unless that which was sown should, by the kindness of Heaven, vegetate? Wherefore he concludes, that he that planteth, and he that watereth is nothing, but that the whole is to be ascribed to God, who alone gives the increase.
1 Corinthians 2:4-5 and my speech and my message were not in plausible words of wisdom, but in demonstration of the Spirit and of power,  that your faith might not rest in the wisdom of men but in the power of God.
John 6:48-58 I am the bread of life.  Your fathers ate the manna in the wilderness, and they died.  This is the bread which comes down from heaven, that a man may eat of it and not die.  I am the living bread which came down from heaven; if any one eats of this bread, he will live for ever; and the bread which I shall give for the life of the world is my flesh."  The Jews then disputed among themselves, saying, "How can this man give us his flesh to eat?"  So Jesus said to them, "Truly, truly, I say to you, unless you eat the flesh of the Son of man and drink his blood, you have no life in you;  he who eats my flesh and drinks my blood has eternal life, and I will raise him up at the last day.  For my flesh is food indeed, and my blood is drink indeed.  He who eats my flesh and drinks my blood abides in me, and I in him.  As the living Father sent me, and I live because of the Father, so he who eats me will live because of me.  This is the bread which came down from heaven, not such as the fathers ate and died; he who eats this bread will live for ever."
It was always held to convey the remission of sins . . . the theory that it mediated the Holy Spirit was fairly general . . . The early view, therefore, like the Pauline, would seem to be that baptism itself is the vehicle for conveying the Spirit to believers; in all this period we nowhere come across any clear pointers to the existence of a separate rite, such as unction or the laying on of hands, appropriated to this purpose.
(Early Christian Doctrines, San Francisco: Harper Collins, revised edition, 1978, 194-195)
St. Clement of Alexandria
When we are baptized, we are enlightened. Being enlightened, we are adopted as sons. Adopted as sons, we are made perfect. Made perfect, we become immortal . . . ‘and sons of the Most High’ [Ps. 82:6]. This work is variously called grace, illumination, perfection, and washing. It is a washing by which we are cleansed of sins, a gift of grace by which the punishments due our sins are remitted, an illumination by which we behold that holy light of salvation.
(The Instructor of Children 1:6:26:1 [A.D. 191])
St. Cyril of JerusalemIf any man does not receive baptism, he does not have salvation. The only exception is the martyrs, who, even without water, will receive baptism, for the Savior calls martyrdom a baptism [Mark 10:38]. . . . Bearing your sins, you go down into the water; but the calling down of grace seals your soul and does not permit that you afterwards be swallowed up by the fearsome dragon. You go down dead in your sins, and you come up made alive in righteousness.
(Catechetical Lectures 3:10, 12 [A.D. 350])
This is the meaning of the great sacrament of baptism, which is celebrated among us: all who attain to this grace die thereby to sin—as he himself [Jesus] is said to have died to sin because he died in the flesh (that is, ‘in the likeness of sin’)—and they are thereby alive by being reborn in the baptismal font, just as he rose again from the sepulcher. This is the case no matter what the age of the body. For whether it be a newborn infant or a decrepit old man—since no one should be barred from baptism—just so, there is no one who does not die to sin in baptism. Infants die to original sin only; adults, to all those sins which they have added, through their evil living, to the burden they brought with them at birth.
(Handbook on Faith, Hope, and Love 13 [A.D. 421]).
St. Gregory of Nyssa
Since, then, that God-containing flesh partook for its substance and support of this particular nourishment also, and since the God who was manifested infused Himself into perishable humanity for this purpose, viz. that by this communion with Deity mankind might at the same time be deified, for this end it is that, by dispensation of His grace, He disseminates Himself in every believer through that flesh, whose substance comes from bread and wine, blending Himself with the bodies of believers, to secure that, by this union with the immortal, man, too, may be a sharer in incorruption. He gives these gifts by virtue of the benediction through which He transelements the natural quality of these visible things to that immortal thing.
(The Great Catechism, chapter XXXVII; NPNF 2, Vol. IV)
St. John Chrysostom
Christ is present. The One who prepared that [Holy Thursday] table is the very One who now prepares this [altar] table. For it is not a man who makes the sacrificial gifts become the Body and Blood of Christ, but He that was crucified for us, Christ Himself. The priest stands there carrying out the action, but the power and grace is of God. “This is My Body,” he says. This statement transforms the gifts.
(Homilies on the Treachery of Judas, 1, 6)
The Catholic church, both Greek and Latin, sees in the Eucharist not only a sacramentum, in which God communicates a grace to believers, but at the same time, and in fact mainly, a sacrificium, in which believers really offer to God that which is represented by the sensible elements. For this view also the church fathers laid the foundation, and it must be conceded they stand in general far more on the Greek and Roman Catholic than on the Protestant side of this question.
(History of the Christian Church, volume 3, § 96. "The Sacrifice of the Eucharist”)